This invention relates to digital communication systems and, more particularly, to synchronization of digital receivers with their associated digital transmitters.
Among the advantages of certain digital communications systems is the ease with which a plurality of information channels can be multiplexed by time division in an array of periodically recurrent time slots or frames. This advantage, however, is not without problems, for transmitters and receivers utilized in such multiplex systems must be maintained in substantially perfect synchronism in order to reconstruct messages correctly. Synchronization of a transmitter and an associated receiver has become known as framing. To this end, it is known to transmit a distinctive pulse pattern in order to maintain the receiver in frame with the transmitter. In many systems, a recurring pulse-no-pulse pattern is employed. It is also very desirable that the receiver rapidly return to an in-frame condition with the transmitter after momentary interruptions in service or any other occurrence that results in a temporary out-of-frame condition. The process of returning to an in-frame condition has become known as reframing.
It has been the usual practice heretofore to employ so-called backward acting reframing systems. The backward acting system is one in which an in-frame condition is obtained and a check is made in each frame to ascertain whether the in-frame condition subsists. To this end, the received signal is compared in a framing detector with a locally generated framing signal pattern. When this comparison indicates a number of rapidly occurring differences between the received framing pattern and the local framing pattern, a hunting action is initiated in order to reframe. In one existing system, an additional pulse is inserted into the local framing signal in each frame which allows the local framing signal to slip a bit each frame relative to the incoming signal. Thus, the local framing signal is compared with each bit position, in turn, of the incoming signal until a comparison is achieved. When the received and locally generated patterns match, the system is in-frame and the hunting action is terminated.
Evaluations of backward acting reframing systems presently in commercial use have indicated that the reframing process requires a substantial period of time. Specifically, it has been determined that in D-1 channel bank receivers the average reframing cycle takes approximately 25 milliseconds. These evaluations have also indicated that many out-of-frame conditions or misframe events are caused by clusters of impulse noise bursts. The individual noise bursts tend to be separated by a time interval comparable to the time required to reframe. A noise burst occurring during the reframing sequence reinitiates the reframing process. Thus, a cluster of noise bursts tends to keep the receiver out-of-frame for a prolonged period. Prolonged out-of-frame periods may cause, among others, noise, service disconnects, system shut downs and the like. Therefore, it is desirable to reframe in the intervals between individual noise bursts.
Another problem with digital receivers utilizing backward acting reframing systems concerns the so-called mass seizure of telephone switching apparatus. During high density error intervals, a bit may be removed from or added into the digital bitstream being supplied to digital receivers associated with a particular telephone switching apparatus. This may cause the framing in all of the receivers to slip in a direction that causes the reframing process in each receiver to sequence through all of the bits in a frame prior to re-establishing an in-frame condition. In some existing receivers, this process would require on average up to approximately 52 milliseconds. Thus, all of the receivers associated with a particular switching apparatus would be out-of-frame for this period. This en masse out-of-frame condition tends to present a so-called off-hook appearance for all channels which, in turn, overloads the switching apparatus, causing it to go out of service. Consequently, all subscribers being serviced by the switching apparatus are disconnected. Such an occurence is extremely undesirable.